1. Field of the Invention
This invention relates to an engine feedback control system and method, and more particularly to such a system and method wherein the feedback control adjusts the air/fuel mixture of the engine.
2. Description of the Related Art
Various control methodology and systems have been employed in conjunction with internal combustion engines so as to improve their performance, particularly in the areas of fuel economy and exhaust emission control. One of the more effective types of controls is a so-called "feedback" control. With this type of control, a basic air/fuel ratio is set for the engine for given engine running parameters. The final adjustment in the air/fuel ratio is made from a sensor that senses the air/fuel ratio in the combustion chamber. Adjustments are then made from the basic setting in order to bring the air/fuel ratio into the desired range.
Normally, the type of sensor employed for such feedback controls is an oxygen (O.sub.2) sensor. By determining the amount of oxygen in the exhaust gases from the combustion chamber, it is possible to fairly accurately measure the actual fuel ratio that was delivered to the combustion chamber.
The system operates on a feedback-control principle, continuously making corrections to accommodate deviations from the desired ratio. Adjustments are made in stepped intervals until the sensor output goes to the opposite sense from its previous signal. For example, if the mixture was running rich, then lean adjustments are made until the mixture strength is sensed to be lean. Adjustments are then made back into the rich direction in order to try to maintain the desired ratio.
These systems have the drawback that adjustments to the air/fuel ratio affect the power output of the engine differently depending on the air/fuel ratio at which the adjustment is made. For example, the same quantitative increase in the air/fuel ratio made to a lean air/fuel mixture as compared to a rich air/fuel mixture will decrease engine power by a greater degree. Adjustments which greatly affect engine power are generally undesirable.
When the air/fuel ratio of the engine shifts the engine torque and especially engine speed also fluctuate. This is particularly noticeable in engine employing lean or fast burn combustion chamber technology. Thus, even if an operator tries to keep a steady throttle position, the engine speed fluctuates. These fluctuations are undesirable as they give the operator an uneasy feeling.